This invention relates to a pipeline system for transporting warm product, such as newly-produced oil, through frost-susceptible ground or soil. In particular, the invention pertains to a method for reducing or eliminating stress on an offshore pipeline caused by thawing of the underlying zone of permafrost.
In arctic regions a mixture of soil, rock, and ice, which is referred to as permafrost, remains in essentially permanently frozen condition downwardly from a depth a few feet below or near the surface. The layer of surface soil above the permafrost commonly referred to as the "active zone" undergoes alternate thawing and freezing during the warm and cold seasons.
The permafrost layer also extends beneath arctic oceans. At a distance from the shore the zone of permafrost usually lies deeply buried under a layer of soil kept thawed by contact with comparatively warm ocean water. For instance, in 100 feet of water the top of the permafrost zone is about 30 to 80 feet beneath the seafloor. But at distances progressively closer to shore the top of the permafrost zone generally lies at progressively shallower depths until at the shoreline a layer of thawed soil only about five feet in depth covers the permafrost.
Construction to develop petroleum resources beneath the Alaskan Beaufort Sea will require pipelines carrying hot oil to land-based facilities to lie along the seafloor, preferably installed in a trough or depression as protection from damage caused by the keels of ice floes or ridges which plow the sea bottom. Ice-bonded permafrost is known to lie buried beneath the seafloor throughout the area of potential development.
In deep water a pipeline running atop the thick layer of thawed soil does not transmit sufficient heat to thaw the deeply buried underlying permafrost. In the transition zone near the shoreline, however, a warm-oil pipeline installed along the overlying soil will transmit sufficient heat downwardly to reach the shallow-lying permafrost. When ice in the permafrost is thawed, the soil shrinks, undergoing an average decrease in volume of 10 percent, and in ice-rich soil the decrease can be as much as 40 percent. The overlying seafloor slumps correspondingly. As a result the pipeline loses support from beneath and slumps along its length. As pipelines may operate at temperatures as high as 200.degree. F., differential settling of the pipeline due to melting of the underlying permafrost is sufficient to cause sections of the pipeline to sink or shift into attitudes that produce localized curvatures in excess of design tolerances. Rupture and oil spills at sea can result.
A similar problem attends pipelines buried across permafrost terrain. On land, however, certain remedies have been taken to prevent slumping and attendant rupture of the pipelines. For instance, a row of heat pipes can be emplanted along each side of the pipeline as disclosed in U.S. Pat. Nos. 3,217,791 and 4,271,681 to Long and Schertz, respectively. Heat pipes comprised of a sealed pipe containing a quantity of low-boiling point liquid are embedded in the soil with their upper ends extending into the atmosphere. Transfer of heat to the colder atmosphere is effected by the change of state of a low boiling point liquid in the bottom of the pipe which absorbs heat from the surrounding soil and evaporates. Vapors so formed move to the top of the pipe and are condensed by the cooler atmosphere, flowing back as liquid to the bottom of the pipe in a continuous cycle of operation. Heat pipes can keep frozen ground frozen, or can freeze thawed ground without causing frost heave. However, heat pipes are not suitable for installation in the seafloor due to its shifting and inconstant state. Another remedy used on land is to elevate the pipeline so that its warmth is harmlessly dissipated into
the atmosphere, as is done with the Trans-Alaskan pipeline. At sea, however, the constant threat of damage from floating ice mandates that the pipeline be placed beneath the level of the seafloor for its security. Yet another remedy used on land is to cover or wrap the buried pipeline in insulation, but insulation only slows the transfer of heat and does not stop it. In time, therefore, thawing will occur.
None of the methods used to prevent melting of the permafrost underlying a buried pipeline on land is suitable for application at sea. What is particularly needed is a method of installing pipelines at sea, particularly pipelines carrying warm fluids such as newly produced oil and gases, so that slumping of the pipeline due to melting of the underlying permafrost is sufficiently minimized that danger of rupture is removed.